1. Field of the Invention
The present invention relates to dual mode filters, and more specifically, to a surface-mounted dual mode filter.
2. Description of the Related Art
Well-known dual mode filters including a filter element that is disposed in a package defined by a casing substrate and a conductive cap have been widely used.
FIG. 18 illustrates the structure of a well-known dual mode filter.
In the dual mode filter, an input electrode 51, an output electrode 52, and a ground electrode 53 are provided on a casing substrate 50. These electrodes 51 to 53 are arranged so as to wrap around the casing substrate 50 from the top surface to the bottom surface. On the top surface of the casing substrate 50, a rectangular-frame shaped insulating layer 54 is provided. As described in Japanese Unexamined Utility Model Application Publication No. 5-25832, a filter element 60 is a piezoelectric resonator defined by connecting a plurality of piezoelectric vibrators via relay capacitors. The filter element 60 includes a composite substrate defined by bonding piezoelectric substrates 61 and 62 having piezoelectric characteristics to a dielectric substrate 63 having dielectric characteristics but no piezoelectric characteristics. Vibrators 64 and 65 are provided on the piezoelectric substrates 61 and 62, respectively. The vibrators 64 and 65 simultaneously generate two oscillation modes, particularly, a symmetric mode (S mode) and an asymmetric mode (A mode), between a pair of segmented electrodes provided on a first principal surface and a counter electrode provided on a second principal surface.
On the top surface of the filter element 60, terminal electrodes 66 and 67 are provided at both ends thereof. A capacitance extraction electrode 68 is provided at a central portion of the top and bottom surfaces of the filter element 60. Since the capacitance extraction electrodes 68 are provided on the dielectric substrate 63, undesired vibrations in a relay capacitance section are prevented from occurring. The terminal electrodes 66 and 67 on the filter element 60 are connected to the input and output electrodes 51 and 52 on the casing substrate 50 via conductive paste 69a and 69b, respectively. The capacitance extraction electrodes 68 are connected to the ground electrode 53 on the casing substrate 50 via a conductive paste 69c. 
After mounting the filter element 60 on the casing substrate 50, silicone rubber 70 is applied to the top surface of the filter element 60. A conductive cap 71 is attached to the insulating layer 54 so as to cover the filter element 60, and the interior is sealed. Accordingly, a surface-mounted dual mode filter is obtained.
In the dual mode filter structured described above, the two vibrators 64 and 65 are integrally provided on one element. Interference between the two vibrators 64 and 65 causes undesired vibrations.
FIG. 19 shows amplitude characteristics of the dual mode filter described above. As shown in FIG. 19, an in-band ripple R is generated by the interference between the vibrators 64 and 65. Further, out-of-band attenuation is relatively small.
In this known structure, two vibrators are provided on one element. Thus, a single composite substrate must be formed by integrally bonding the piezoelectric substrates 61 and 62 and the dielectric substrate 63. As a result, the manufacturing costs are increased, and the reliability of the electrodes at the boundary between the substrates is reduced.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a dual mode filter in which undesired vibrations due to interference between vibrators are prevented from occurring, which has greatly reduced manufacturing costs, and which has greatly increased connection reliability.
According to a first preferred embodiment of the present invention, a dual mode filter includes a casing substrate on which an input electrode, an output electrode, two ground electrodes, and a relay electrode are provided, first and second filter elements mounted on the casing substrate, and a cap connected to and fixed on the casing substrate so as to cover the filter elements. Each of the first and second filter elements includes a first terminal electrode at a first end of a first principal surface of a piezoelectric substrate, a second terminal electrode in a central portion of the piezoelectric substrate, a pair of segmented electrodes provided in a portion of the piezoelectric substrate between the first and second terminal electrodes and which are connected to the first and second terminal electrodes, respectively, a counter electrode provided on a second principal surface of the piezoelectric substrate and which is opposed to the segmented electrodes to define a dual mode vibrator, and a third terminal electrode provided at a second end of the piezoelectric substrate and which is connected to the counter electrode. The first terminal electrode of the first filter element is connected to the input electrode on the casing substrate via a conductive bonding material, and the third terminal electrode is connected to one ground electrode on the casing substrate via a conductive bonding material. The first terminal electrode of the second filter element is connected to the output electrode on the casing substrate via a conductive bonding material, and the third terminal electrode is connected to the other ground electrode on the casing substrate via a conductive bonding material. The second terminal electrode of the first filter element is connected to the relay electrode on the casing substrate via a conductive bonding material, and the second terminal electrode of the second filter element is connected to the relay electrode on the casing substrate via a conductive bonding material.
According to preferred embodiments of the present invention, instead of providing two vibrators on one element, one dual-mode vibrator is provided on each of the two elements. These two elements are spaced a desired distance from each other on the casing substrate and are connected to each other through the relay electrode on the casing substrate. Thus, undesired vibrations due to interference between the two vibrators are prevented.
It is unnecessary to form a composite substrate by integrally bonding a piezoelectric substrate and a dielectric substrate. Thus, the manufacturing costs are greatly reduced. The dual mode filter according to preferred embodiments of the present invention does not have reduced reliability of electrodes at the boundary between the two substrates.
Since the relay electrode is provided on the casing substrate, the relay electrode is not opposed to the filter elements with the ground electrodes and the piezoelectric substrate therebetween. Thus, undesired vibrations are efficiently suppressed.
Preferably, the first and second filter elements are mounted such that each first terminal electrode, each second terminal electrode, and each pair of segmented electrodes face the casing substrate side. In other words, preferably, the first and second filter elements are mounted such that each counter electrode and each third terminal electrode face upward.
When connecting the first terminal electrodes of the filter elements to the input electrode and the output electrode by the conductive bonding materials and when connecting the second terminal electrodes to the relay electrode via the conductive bonding materials, with these electrodes facing upward (opposite to the casing substrate side), the conductive bonding material, such as a conductive paste or soldering, must continuously be applied to portions from the terminal electrodes on the top surface of each filter element to the electrodes on the casing substrate. In particular, it is difficult to connect the second terminal electrodes to the relay electrode. In other words, because each second terminal electrode is provided in a central portion of each filter element, the second terminal electrode is near the vibrator. When a large amount of conductive bonding material is applied, the conductive bonding material may reach the vibrator.
In contrast, when the first terminal electrodes and the second terminal electrodes are mounted to face toward the casing substrate side, the second terminal electrodes is connected face-to-face to the relay electrode on the casing substrate via the conductive bonding material. Thus, the connection is simplified, and the connection reliability is greatly improved.
Preferably, the second terminal electrodes of the first and second filter elements are each arranged at a position near a first edge of the first principal surface of the piezoelectric surface so as not to oppose the electrodes provided on the second principal surface of the piezoelectric substrate, and the first filter element and the second filter element are arranged next to each other on the casing substrate such that the second terminal electrodes are opposed to each other.
When each second terminal electrode has a width that is substantially equivalent to the width of each filter element, the second filter element is opposed to the lead electrode connecting the counter electrode and the third terminal electrode provided on the second principal surface with the piezoelectric substrate therebetween. In the opposed portion, undesired vibrations may occur.
In order to prevent the second terminal electrode from being opposed to the electrodes provided on the second principal surface, the second terminal electrode is provided at a position near the first edge of the first principal surface of the piezoelectric substrate, thus preventing undesired vibrations from occurring. The first and second filter elements are arranged next to each other on the casing substrate such that the second terminal electrodes, which are provided at the positions near the edges, are opposed to each other, thus the length of the dual mode filter according to preferred embodiments of the present invention is greatly reduced. Further, the relay electrode is shortened, and hence the width of the dual mode filter is reduced.
As a result, the size of the compact dual mode filter is greatly reduced.
Preferably, a thickness Ta of the first filter element differs from a thickness Tb of the second filter element, or a width Wa of the first filter element differs from a width Wb of the second filter element.
Vibrating electrodes of each filter element preferably include the segmented electrodes and the counter electrode opposed thereto. Vibrations are generated between these electrodes, thereby achieving desired filter characteristics. Actually, however, undesired vibrations occur between the counter electrode and the lead electrode connecting the segmented electrode and the terminal electrode. Also, undesired vibrations occur between the segmented electrodes and the lead electrode connecting the counter electrode and the third terminal electrode. These undesired vibrations are thickness mode vibrations and appear as ripples on the main waveform of the filter.
By making at least one of the thickness and the width of the first filter element different from that of the second filter element, a frequency at which undesired vibrations occur in one element is different from that in the other element. Thus, interference between the undesired vibrations is effectively prevented, and ripples are greatly reduced as compared with a case in which the thickness and the width of one filter element are the same as those of the second filter element.
The thickness Ta of the first filter element and the thickness Tb of the second filter element preferably satisfies one of the following inequalities:
Tb less than Ta less than 1.01Tb, and 
0.99Tb less than Ta less than Tb. 
In other words, by maintaining the difference in thickness between the two filter elements within approximately 1%, the difference between the resonant frequencies of the two filter elements is maintained within approximately 1%. When the difference between the resonant frequencies of the two filter elements is greater than or equal to about 1%, the band width ratio (pass band width/fo) decreases suddenly, and the balance between a symmetric mode (S mode) and an asymmetric mode (A mode) produced by the two elements substantially deteriorates. As a result, the main waveform is segmented.
The width Wa of the first filter element and the width Wb of the second filter element preferably satisfies one of the following inequalities:
Wb less than Wa less than 1.05 Wb, and 
0.9Wb less than Wa less than Wb 
In the width mode, ripples periodically occur at a ratio between the element thickness and the element width, and an appropriate width region periodically occurs. By changing the width while keeping the difference in width between the two elements within approximately 5%, ripples are efficiently reduced.
A distance La1 from the midpoint between the segmented electrodes of the first filter element to a first end in the longitudinal direction, a distance La2 from the midpoint to a second end in the longitudinal direction, a distance Lb1 from the midpoint between the segmented electrodes of the second filter element to a first end in the longitudinal direction, and a distance Lb2 from the midpoint to a second end in the longitudinal direction preferably satisfies one of the following inequalities:
0 less than |La1xe2x88x92Lb1| less than 2t, and 
0 less than |La2xe2x88x92Lb2| less than 2t, 
wherein t represents the greater of the thickness Ta of the first filter element and the thickness Tb of the second filter element.
Propagating waves which are generated by the vibrating electrodes and which propagate in the longitudinal direction are reflected from a narrow-side edge of each filter element and interfere with other vibrations, thus causing large ripples and spurious responses. By changing the distance from the vibrating electrodes to the narrow-side edge, a frequency at which undesired vibrations occur in one filter element is different from that in the other filter element. Since there is no interference between the undesired vibrations, ripples and spurious responses are greatly reduced as compared with a case in which the distances La1, La2, Lb1, and Lb2 are the same.
By changing the ratio between La1:La2 and Lb1:Lb2, the advantages are even greater.
A distance Ha1 between the first lead electrode of the first filter element and a first edge of the first filter element in the longitudinal direction and a distance Ha2 between the second lead electrode of the first filter element and the first edge of the first filter element in the longitudinal direction preferably differs from a distance Hb1 between the first lead electrode of the second filter element and a first edge of the second filter element in the longitudinal direction and a distance Hb2 between the second lead electrode of the second filter element and the first edge of the second filter element in the longitudinal direction, respectively.
As described above, when undesired thickness-mode vibrations occur, the vibrations are reflected from a long-side edge of each element and interfere with other vibrations, thus causing large ripples and spurious responses. By changing the positions of the lead electrodes of the two filter elements, a frequency at which undesired vibrations occur in one filter element is different from that in the other filter element. Hence, the interference between the undesired vibrations is prevented, and ripples and spurious responses are greatly reduced.
The distance Ha1, the distance Ha2, the distance Hb1, the distance Hb2, and the width W of one of the first and second filter elements may satisfy the following relationship:
0 less than |Ha1xe2x88x92Hb1| less than W/4, and 
0 less than |Ha2xe2x88x92Hb2| less than W/4, 
wherein W represents the greater of the width Wa of the first filter element and the width Wb of the second filter element.
Specifically, there are cases where strong vibrations are caused by the lead electrodes, whereas in other cases weaker vibrations are caused by the lead electrodes. This depends upon the positions of the lead electrodes of the filter elements. Thus, it is important to adjust the positions of the lead electrodes of the two filter elements within an area in which weaker vibrations are caused by the lead electrodes. By maintaining the difference between the lead electrodes within about W/4, ripples are efficiently suppressed.
A distance Ja between the third lead electrode of the first filter element and a first edge of the first filter element in the longitudinal direction preferably differs from a distance Jb between the third lead electrode of the second filter element and a first edge of the second filter element in the longitudinal direction.
In this case, as described above, undesired thickness-mode vibrations are reflected from a long-side edge of each filter element and interfere with other vibrations, thus causing large ripples and spurious responses. By changing the positions of the third lead electrodes of the two elements, a frequency at which undesired vibrations occur in one filter element is different from that in the other filter element, and ripples and spurious responses are greatly reduced.
The distance Ja, the distance Jb, and the width W of one of the first and second filter elements preferably satisfy the following relationship:
0 less than |Jaxe2x88x92Jb| less than W/4 
wherein W represents the greater of the width Wa of the first filter element and the width Wb of the second filter element.
As described above, there are cases in which strong vibrations are caused by the lead electrodes, whereas in other cases weaker vibrations are caused by the lead electrodes. This depends upon the positions of the lead electrodes of the filter elements. Thus, it is important to slightly adjust the positions of the lead electrodes of the two filter elements within an area in which weaker vibrations are caused by the lead electrodes. By maintaining the difference between the lead electrodes within about W/4, ripples are efficiently suppressed.
The first and second lead electrodes and the third lead electrode of the first filter element are preferably provided at unopposed positions on top and bottom surfaces of the first filter element. The first and second lead electrodes and the third electrode of the second filter element are preferably provided at unopposed positions on top and bottom surfaces of the second filter element. A distance Da in the width direction between the first and second lead electrodes and the third lead electrode of the first filter element preferably differs from a distance Db in the width direction between the first and second lead electrodes and the third lead electrode of the second filter element.
Even when the lead electrode connecting the segmented electrode and the terminal electrode is not directly opposed to the lead electrode connecting the counter electrode and the terminal electrode, undesired vibrations occur due to stray capacitance. These vibrations are related to the thickness and produce ripples on the main waveform of the filter. By making the distance between the lead electrodes on the top and bottom surfaces of one filter element different from that in the other filter element, a frequency at which undesired vibrations occur in one filter element is different from that in the other filter element. Since these vibrations do not interfere with each other, the size of the ripples is greatly reduced as compared with a case in which the distance between the lead electrodes of one filter element is the same as that in the other filter element.
The casing substrate is preferably a dielectric multilayer member including a plurality of capacitance electrodes provided therein. At least one of the capacitance electrodes is connected to the relay electrode, and at least another one of the capacitance electrodes is connected to the ground electrodes.
In order to adjust filter characteristics, conventionally, a relay capacitor is connected between two dual mode vibrators. Since the relay capacitor is provided on an integral piezoelectric substrate, undesired vibrations occur. In order to solve this problem, a discrete capacitor component is mounted on the casing substrate and defines the relay capacitor. However, with this arrangement, the number of nodes is increased, and the reliability is reduced.
In contrast, according to preferred embodiments of the present invention, the relay capacitor is provided in the interior of the casing substrate. Unlike connecting a discrete component, connections are unnecessary, and hence the reliability is greatly improved. Since the relay capacitor is a built-in capacitor in the casing substrate, short-circuiting of the relay capacitor with other circuits is effectively prevented. Since the casing substrate has an area that is larger than that of each filter element, the area of a portion in which the capacitance electrodes are opposed to each other is increased, and an arbitrary relay capacitance is obtained.
Preferably, the casing substrate is quadrangular. The input electrode and the output electrode are provided at opposing corners of the casing substrate. The two ground electrodes are provided at the remaining opposing corners. The third terminal electrode of the first filter element is connected to one ground electrode on the casing substrate. The third terminal electrode of the second filter element is connected to the other ground electrode on the casing substrate.
By providing the outer electrodes at four corners of the casing substrate, the size of the casing substrate is greatly reduced. By arranging the input electrode and the output electrode at opposing corners, an influence of the stray capacitance generated between the input and the output is greatly reduced. Out-of-band attenuation is increased, and short-circuiting between the input and output electrodes is prevented.
According to preferred embodiments of the present invention, since two filter elements each having a single dual mode vibrator are mounted on a casing substrate with a separation therebetween and are connected to electrodes on the casing substrate, interference between the two vibrators is prevented, and undesired vibrations are prevented from occurring. Since two small filter elements are individually manufactured and mounted on the casing substrate, the manufacturing costs are greatly reduced as compared with a known dual mode filter using a composite substrate formed by integrally bonding a piezoelectric substrate and a dielectric substrate. Also, the dual mode filter according to preferred embodiments of the present invention does not suffer from the problem of reduced reliability of electrodes at the boundary between the two substrates. Since a relay electrode is provided on the casing substrate at a portion spaced from the filter elements, the relay electrode and the filter elements are not opposed to each other with ground electrodes and piezoelectric substrates therebetween. Thus, undesired vibrations are effectively prevented from occurring.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments thereof with reference to the attached drawings.